Cucumber (Cucumis sativus L.), sensitive to cold stress, is one of the most economically important vegetables. Here, we systematically investigated the roles of exogenous glycine betaine, chitosan, and chitosan oligosaccharide in alleviating cold stress in cucumber seedlings. The results showed that 50 mg·L⁻¹ chitosan oligosaccharide had the best activity. It effectively increases plant growth, chlorophyll content, photosynthetic capacity, osmotic regulatory substance content, and antioxidant enzyme activities while reducing relative electrical conductivity and malondialdehyde levels in cucumber seedlings under cold stress. To reveal the protective effects of chitosan oligosaccharide in cold stress, cucumber seedlings pretreated with 50 mg·L⁻¹ chitosan oligosaccharide were sampled after 0, 3, 12, and 24 h of cold stress for transcriptome analysis, with distilled water as a control. The numbers of differentially expressed genes in the four comparison groups were 656, 1274, 1122, and 957, respectively. GO functional annotation suggested that these genes were mainly involved in “voltage-gated calcium channel activity”, “carbohydrate metabolic process”, “jasmonic acid biosynthetic”, and “auxin response” biological processes. KEGG enrichment analysis indicated that these genes performed important functions in “phenylpropanoid biosynthesis”, “MAPK signaling pathway—plant”, “phenylalanine metabolism”, and “plant hormone signal transduction.” These findings provide a theoretical basis for the use of COS to alleviate the damage caused by cold stress in plant growth and development.

Cucumbers (Cucumis sativus L.) are a global popular vegetable and are widely planted worldwide. However, cucumbers are susceptible to various infectious diseases such as Fusarium and Verticillium wilt, downy and powdery mildew, and bacterial soft rot, which results in substantial economic losses. Grafting is an effective approach widely used to control these diseases. The present study investigated the role of wax and the phenylpropanoid biosynthesis pathway in black-seed pumpkin rootstock-grafted cucumbers. Our results showed that grafted cucumbers had a significantly higher cuticular wax contents on the fruit surface than that of self-rooted cucumbers at all stages observed. A total of 1132 differently expressed genes (DEGs) were detected in grafted cucumbers compared with self-rooted cucumbers. Pathway enrichment analysis revealed that phenylpropanoid biosynthesis, phenylalanine metabolism, plant circadian rhythm, zeatin biosynthesis, and diterpenoid biosynthesis were significantly enriched. In this study, 1 and 13 genes involved in wax biosynthesis and the phenylpropanoid biosynthesis pathway, respectively, were up-regulated in grafted cucumbers. Our data indicated that the up-regulated genes in the wax and phenylpropanoid biosynthesis pathways may contribute to disease resistance in rootstock-grafted cucumbers, which provides promising targets for enhancing disease resistance in cucumbers by genetic manipulation.

Cucumber is an important vegetable crop, and grafts often affect the quality and wax loss in cucumber fruit and affect its value. However, their metabolites and molecular mechanisms of action remain unclear. Metabolome and transcriptome analyses were conducted on the fruit peels of self-rooted plants (SR) grafted with white seed pumpkin (WG). The results showed that there were 352 differential metabolites in the fruit peels of the SR and WG. The transcriptome analysis showed 1371 differentially expressed genes (DEGs) between the WG and SR. These differentially expressed genes were significantly enriched in plant hormone signal transduction, cutin, suberin, wax biosynthesis, phenylpropanoid biosynthesis, and zeatin biosynthesis. By analyzing the correlation between differential metabolites and differentially expressed genes, six candidate genes related to the synthesis of glycitein, kaempferol, and homoeriodictyol were identified as being potentially important. Key transcription factors belonging to the TCP and WRKY families may be the main drivers of transcriptional changes in the peel between the SR and WG. The results of this study have provided a basis for the biosynthesis and regulation of wax loss and quality in grafted cucumbers and represents an important step toward identifying the molecular mechanisms of grafting onto cucumber fruit.